1. Field of the Invention
This invention relates to a method of manufacturing an IGFET having metal-insulating film-semiconductor construction, and more particularly to a method of manufacturing an insulated gate field effect transistor (IGFET), in which a predetermined amount of impurity is introduced into the insulating film to produce immobile charges to control the carrier concentration in the surface of the semiconductor and alter the conductivity type, and to a method developed from the above one which facilitate fabrication of an IGFET in which a transistor of depletion mode is coupled as a load with a transistor of enhancement mode.
2. Description of the Prior Art
In the prior art, the IGFET's are classified into N-channel and P-channel types according to the carrier conductivity type and are also divided into those which conduct at zero bias (depletion mode) and those which do not conduct at zero bias (enhancement mode) according to the mode of operation.
In the transistor of enhancement mode of operation, the gate bias and the drain bias are of the same polarity, so that direct interstage coupling is possible and various integrated circuits are produced utilizing this property. It is advantageous in these integrated circuits that the load is a transistor, which is normally in an off state, and accordingly power loss is relatively small. Where the load is the transistor of depletion mode, it is possible to improve the switching characteristic by interconnecting its gate and source electrodes to hold the transistor in the on state at all times.
Silicon dioxide formed on the surface of a silicon semiconductor tends to make the surface a little N-type and the P-channel type transistor of enhancement mode is easy to produce but the P-channel type transistor of depletion mode is difficult to produce.
One method for manufacturing the transistor of depletion mode is to diffuse boron through a gate oxide film into an N-type silicon substrate by an ion implantation method to lower the concentration in the surface of the substrate to form a P-channel thin; but an ion implantation device is extremely expensive and its production is very difficult to control. Another method is to induce the P-channel by using as a gate insulating film an alumina film including negative charges therein; but, this method is less than desirable because of the combined use of an oxide film with the alumina film, the difficulty in the control of the film thickness and the unsatisfactory reproducibility due to difficulty in the control of the charge in the film.